The present invention relates to a birefringence measurement apparatus for calculating a retardation magnitude and an azimuth of a principal axis of an object retardance to be measured, a strain remover having the birefringence measurement apparatus, a polarimeter or polarization detector that includes the birefringence measurement apparatus, and an exposure apparatus that includes the polarimeter.
As the need for smaller and thinner electronic apparatuses grows in recent years, finer semiconductor devices mounted in these electronic apparatuses have been increasingly demanded, and various proposals have been made for higher exposure resolution to fulfill this demand.
Since a shortened wavelength of an exposure light source is one effective means for higher resolution, the recent exposure light source has shifted from a g-line (with a wavelength of about 436 nm) and an i-line (with a wavelength of about 365 nm) to a KrF excimer laser (with a wavelength of about 248 nm) and an ArF excimer laser (with a wavelength of about 193 nm). In the near future, use of an F2 excimer laser (with a wavelength of approximately 157 nm) is expected to be promising.
A conventional optical element is available to an optical system down to a wavelength region for the i-line, but conventional optical glass cannot be used for such a wavelength region as covers the KrF and ArF excimer lasers and the F2 laser due to its low transmittance. Therefore, an optical system in an exposure apparatus that uses the excimer laser as a light source has commonly used an optical element made of quartz glass (SiO2) or calcium fluoride (CaF2) having larger transmittance to light with a shortened wavelength, and it has been considered that exposure apparatus that uses the F2 laser as a light source necessarily uses an optical element made of calcium fluoride.
Calcium fluoride single crystal has been manufactured mainly by a crucible descent method or Bridgman method. This method fills highly purified materials of chemical compounds in a crucible, melts in a growth device, and gradually descends the crucible, thereby crystallizing the materials from the bottom of the crucible. The heat history in this growth process remains as a stress in calcium fluoride crystal. Calcium fluoride exhibits birefringence to the stress. The residual stress deteriorates optical performance. In order to reduce the retardation magnitude as little as possible which results from the existing residual stress or strain amount, a heat treatment has been conventionally used to reduce or remove the residual stress from the optical element.
The strain removing method heats up an optical element under a desired condition up to the preset temperature of viscous fluidity region where the optical element exhibits structurally viscous fluid flows viscously and structural change, holds the preset temperature for a predetermined period of time to mitigate temporary strain due to permanent strain and rapid rise in temperature, and then gradually cools the optical element under a gradual change condition that may maintain the mitigated strain down to temperature which does not provide a structural change, followed by natural cooling.
Birefringence is one influential factor to imaging performance of an exposure apparatus. As elucidated by NIST's publication in May of 2001, calcium fluoride, which is used for an optical system in an exposure apparatus that utilizes ArF excimer laser, F2 laser, etc. as an exposure light source, includes intrinsic birefringence that results from its crystal structure, in addition to stress birefringence that results from an internal stress (or stress strain). Therefore, cares for birefringence including the intrinsic birefringence becomes critical in developing an exposure apparatus. They require a grasp of the retardation magnitude of the exposure wavelength. In addition, a measurement of the residual birefringence in the optical element is essential to the strain removal. A conventional birefringence measurement method includes a rotary analyzer method, a phase compensation method that utilizes a Babinet compensator, etc., a Senarmont method that utilizes a quarter-wave plate, a phase modulation method that utilizes a photoelastic modulator (“PEM”), and an optical heterodyne method that uses a Zeeman laser etc. as a light source.
A demand for the reduced residual stress in the optical element has become increasingly strict for recent more precise optical systems and denser semiconductor devices, and the conventional birefringence measurement methods have faced difficulties in measuring the residual birefringence in the optical element with satisfactory precision. In addition, the conventional birefringence measurement methods cannot easily measure birefringence characteristics in an ultraviolet region of calcium fluoride etc., when using ultraviolet light as a measurement light source, due to the unstable light source and the manufacture difficulties of a PEM and a phase shifter, such as a quarter-wave plate for a measurement wavelength. Moreover, long-term research and development activities are necessary to establish heat treatment conditions of optical elements, and the long-term heat treatment are required to reduce the residual stress in the optical element. This deteriorates the productivity of the optical element, and causes an increase of production cost.